If Biftu gives me a penny, I will have twice as many pennies as he has. If I give Biftu a penny, he and I will have a total of 15 pennies. How many pennies do I currently have?
+128473 Let the number of pennies you have = x
Let the number of pennies that Bill has = y
So x + y = 15 (1)
And (x + 1) = 2 ( y -1) ⇒ x + 1 = 2y - 2 ⇒ x = 2y - 3 (2)
Sub (2) into (1) and we get that
2y - 3 + y = 15
3y = 18
y = 6 = the number Bill has
x + 6 = 15
x = 9 = the number you have
